1. The Field of the Invention
The invention relates to a method and a tool or a tool set in each case for producing a thread in at least two working steps.
2. Background and Relevant Art
For thread production or thread rework, exclusively cutting, exclusively chipless and both cutting and chipless methods and threading tools are known. Cutting thread production is based on the removal of the material of the workpiece in the region of the thread turn (or, thread profile). Chipless thread production is based on production of the thread turn in the workpiece by pressure and the forming or plastic deformation, effected as a result, of the workpiece.
An advantage of the chipless thread production compared with the cutting thread production is that, due to the consolidation or compaction at the surface, the hardness of the material in the region of the thread profile increases and thus a more wear-resistant thread can be produced. Coming within the scope of cutting thread production are taps (cf. Handbuch der Gewindetechnik und Frästechnik [Manual of threading practice and milling practice], publisher: EMUGE-FRANKEN, publishing firm: Publicis Corporate Publishing, year of publication: 2004 (ISBN 3-89578-232-7), designated below only as “EMUGE manual”, chapter 8, pages 181 to 298) and thread milling cutters (cf. EMUGE manual, chapter 10, pages 325 to 372).
A tap is a thread-cutting tool which works axially relative to its tool axis and whose cutting edges are arranged along an external thread having the thread pitch of the thread to be produced. During the production of a thread, the tap is moved with an axial feed and while rotating about its tool axis into a hole of the workpiece at a rotary speed dependent on the feed rate, its cutting edges being permanently in engagement (continuous cut) with the workpiece at the wall of the hole.
In the thread milling cutter, a plurality of milling teeth having milling cutting edges are arranged offset along the tool circumference and/or axially relative to the tool axis. To produce the thread, the thread milling cutter is rotated about its own tool axis and is moved with its tool axis in a linear feed movement on the one hand and additionally in a circular movement about a center axis of the thread to be produced or of the pilot hole in the workpiece on the other hand, as a result of which a helical movement of the tool is obtained, the pitch of which corresponds to the thread pitch of the thread to be produced. The milling cutting edges of the thread milling cutter engage intermittently one after the other in the workpiece (interrupted cut).
Coming within the scope of chipless thread production tools are “thread formers” (cf. EMUGE manual, chapter 9, pages 299 to 324) and “circular thread formers”.
On a tool shank, thread formers have an outer profile which encircles the tool axis spirally or helically with the pitch of the thread to be produced and has an approximately polygonal cross section. The generally rounded-off polygon corner regions form pressing lobes or forming teeth or forming wedges which press the thread into the material by plastic deformation and flow of the workpiece material into the spaces between the outer profile on the one hand and compaction of the workpiece material on the other hand. To produce an internal thread in an already existing hole, the thread former is inserted into the hole with a linear feed movement axially relative to the tool axis and with the tool rotating about this tool axis. Known exemplary embodiments of such (axial) thread formers are also found in DE 101 36 293 A1, DE 199 58 827 A1 or also in DE 39 34 621 C2.
WO 02/094491 A1 discloses a circular thread former and a circular forming method for chipless thread production. This known circular thread former is elongated and comprises a working region with one or more annular circumferential profiles separated from one another by annular grooves and having a least three respective pressing lobes like a polygon. This tool is inserted into a hole having a larger diameter than the tool and performs, in addition to a rotation about its own tool axis, a circular movement along the circumference of the hole and at the same time a linear feed movement into the hole, thereby forming the thread in the hole in a helical movement and in a chipless manner. A further circular former and a further circular forming method are known from DE 103 18 203 A1.
Finally, combined methods having at least two working steps for producing internal threads are known, in which, in a first working step, a preliminary thread is produced and, in a further working step, the internal thread is completed by forming from the preliminary thread. Due to this two- or multi-stage method, the entire volume of the thread profile does not have to be produced by a single forming tool by plastic deformation of the material, and the loading and the wear of the forming tool are reduced. Advantages in the case of coarse thread pitches, materials that flow poorly, the consolidation of thread regions and the smoothing of thread surfaces can be achieved by the combination of cutting and forming production steps.
For such a combined method having a plurality of working steps, a separate tool can be used in a first variant in each working step. DE 10 2004 033 772 A1 discloses such a method, in which, in a first working step, a preliminary thread is initially produced in the workpiece with a cutting or chipless action by means of a tap, thread milling cutter, by turning, grinding or winding or also by means of a thread former and, in a second working step, the finish production of the preliminary thread is effected with a chipless action using a thread former or circular former, the forming wedges of which press into the thread root of the preliminary thread. The forming wedges are in this case centered by widened portions of their flanks in the initially produced thread.
The thread flanks of the preliminary thread produced in the first working step remain unchanged in the second working step, since the widened portions of the forming wedge flanks bear only against the preliminary thread flanks for centering, but do not deform the latter. In the second working step, the workpiece material is deformed further only in the region of the thread root, connecting the two thread flanks, of the preliminary thread. As a result, in the second working step, the entire thread root and also those regions of the thread flanks of the final thread which directly adjoin the thread root and lie in extension of the thread flanks of the preliminary thread are completed by forming and are thus additionally compacted and consolidated, whereas the predominant region of the thread flanks of the final thread have already been completely produced in the first working step and have not been worked further in the second working step.
In a second known variant of a combined method having a plurality of working steps, a combination tool having a cutting tapping part and a chipless thread-forming part, offset axially to the tool axis relative to the tapping part, on a tool shank is used, the tapping part, with rotation about the tool axis and with axial feed, cutting a preliminary thread in the workpiece in a first working step, and the following thread-forming part forming the preliminary thread in a predetermined manner in a second working step in order to produce the final thread. The tap therefore initially cuts the thread and the thread former subsequently partly forms the initially cut thread. Such an axial combination tool and method are known from DE 70 17 590 U and DE 196 49 190 C2.
According to DE 196 49 190 C2 the thread is first of all produced with a precise profile and accurately to size in the thread flanks by a cutting action by means of the tap of the combination tool and then only the thread root of this initially cut thread is compacted with a chipless action to a predetermined final diameter by the directly following thread former. As a result, in particular the first thread turns, following an initial thread cut, can be produced in such a way that they are more resistant to vibrations and are less susceptible to fracture. In the second working step, the two thread flanks cut by the tap in the first working step remain completely unchanged. The workpiece material at the thread root is not made to flow, but is first cut and then only compacted.
DE 70 17 590 U1 discloses a combination tool for producing internal threads, having a tapping drill, designed as a taper or second tap, as a front tool part for the initial cutting of the thread and a thread former, adjoining in the working direction, as a rear tool part for the finishing of the thread.